1. Field of the Invention
The present invention relates to an optical pickup and an optical disc apparatus, and more particularly, to an optical pickup and an optical disc apparatus in which servo control can be stably exerted with a simple configuration.
2. Description of the Related Art
In recent years, high-density and large-capacity optical discs, such as DVDs (Digital Versatile Discs), have been put in practical use and popularized as information storage media capable of storing large volumes of information such as moving pictures.
In optical pickups of known optical disc apparatuses that record or read information on optical discs, a light beam is applied onto an optical disc, and is reflected by an information recording surface of the optical disc. The reflected light beam is received by a photodetector having a plurality of regions, and a tracking error signal is detected by, for example, a push-pull method on the basis of signals output from the photodetector corresponding to the light received by the regions.
However, in the push-pull method using only one beam, detection error of the tracking error signal is sometimes caused by lens shifting.
Accordingly, various techniques of reducing the detection error of the tracking error signal have been proposed. In a so-called differential push-pull method as an example, a main beam and two sub-beams are shifted from one another by a preset amount along a direction orthogonal to tracks. A tracking error signal obtained from the main beam serves as a first push-pull signal, and a tracking error signal obtained from the two sub-beams serves as a second push-pull signal. A tracking error signal is obtained by subjecting the first and second push-pull signals to differential operation.
That is, according to the differential push-pull method, the influence of lens shifting is cancelled, and a tracking error signal can be detected precisely.
Japanese Unexamined Patent Application Publication No. 2004-281026 discloses another technique. In this technique, a part of a light beam that is not diffracted by a track structure of an optical recording medium is divided in two (or four) in the radial direction of the optical recording medium, and the light beam is diffracted by a diffraction grating, and is guided to a photodetector. By detecting the difference in the light intensity in the radial direction, shifting of an objective lens is detected.